6- or 8-Inch Balancer?
Is there a rule of thumb on whether to use a 6- or 8-inch balancer on different Chevy small-blocks? I believe the 6-inch-od balancer was used on the 283 and the 327s. Is this correct? I have used the 6-inch on my race motor for lighter weight and it seems to work fine.
Phil Sprague
St. Helens, OR

2/9These high-end race dampers are modular units that can be custom-tuned for a specific application and rpm range. The design shown here uses internal clutches and springs.

An engine's crankshaft may seem like it's a heavy, homogeneous, solid-metal mass, but in reality a crank bends and twists in response to the torque loads put on it by the pistons and rods. According to Chevrolet sources, "Tests have shown that the crankshaft in a 350ci racing engine can deflect 0.3 degrees at 8,000 rpm." Other authorities claim crank deflections of as much as 2 degrees. When the torque load on the crank is released and the crank untwists, vibrations occur. At certain engine speeds, the torque inputs become synchronized with the vibrations in the crank, resulting in what's known as resonance. Somewhat akin to a tuning fork, this resonance further amplifies the crank's vibrations, causing extreme stress that in a worst-case scenario can result in outright crankshaft failure.

Absent complete crank failure, torsional vibrations can also adversely affect cam timing by transferring the harmonics through the timing chain into the valvetrain. Since the distributor is driven off the cam, ignition performance and spark timing might also be degraded. Other problems include broken drivebelts, fractured oil pumps, and manual trans shift difficulties at certain rpm points. Rod and main journal bearing wear can be accelerated, which in itself can contribute to eventual crank failure.

A short, stiff crank is less affected by torsional vibrations than a long, flexible crank. Within a given crank length (a crank for the same engine family), stiffness is a function of stroke length, the amount of main and rod journal overlap, and crankshaft material. A 3.0-inch-stroke 283 or 302 crank is stiffer than a 3.25-inch-stroke 327 crank, which is stiffer than a 3.48-inch-stroke 350 crank. Forged steel generally makes for a stronger, stiffer crank compared with cast nodular iron.

An engine's rpm operating range also factors into the balancer equation. A drag race engine sees rapid rpm changes for only a quarter-mile at a time-there's not much time for the crank to vibrate. At the other extreme, oval-track engines can spend hours running at high rpm in a relatively narrow band close to the crank's critical resonance frequency. And even a street engine can spend hours cruising at a sustained rpm.

Chevy took all these factors into account when it spec'd harmonic dampers. A forged-crank 283 had little more than a harmonic balancer hub, while those 283s equipped with a nodular iron crank had a real 63/4-inch (not 6-inch) balancer. Low-performance 327s tended to use the smaller 63/4-inch balancers, while the high-performance or Corvette motors (because they were driven aggressively and at higher rpm) used 8-inch balancers. Nearly all 350s came with an 8-inch balancer because of their relatively long stroke. An 8-inch balancer was used on the short-stroke, 3.0-inch-stroke, 302 Z/28 engines because those mills were intended for road racing. The 400 small-block, with its 3.75-inch stroke, used a unique 8-inch balancer with external balance weights. Also, as a group, trucks tended to use the larger balancers because they were intended to carry cargo loads or be used for towing, operating cycles that over time impart extended stress on the engine's bottom end.

Also note that, at least as originally equipped, there were many different factory balancer variations, even on those with the same damper diameter. The original balancers were tuned to the specific engine, rpm range, and stroke they were designed for. Even fore and aft thickness may vary.

Bearing all this in mind, the theory behind using a small-diameter balancer (or even no balancer at all) on a drag-race-only motor is that the engine will accelerate quicker through its rpm range, just as it does by going to a lighter flywheel at the opposite end of the crank. It is thought this should translate into a better elapsed time. But better is a relative term. At best, a light balancer might be worth 0.02 second on a 9-second car.

On the other hand, tests conducted by David Vizard showed that the same 350 engine made more power on an engine dyno when equipped with an 8-inch factory balancer originally used on high-performance 302 and 327 engines (PN 3817173) than it did with just a lightweight aluminum hub. In his book How to Build & Modify Chevrolet Small-Block V-8 Pistons, Rods & Crankshafts (Motorbooks International, 1991, ISBN 0879385790, no longer in print), Vizard said the tests were conducted "under accelerating conditions-not static conditions-and took into account the additional weight of the damper." The large damper probably performed better because it reduced adverse torsional stresses transferred through the timing chain into the camshaft, valvetrain, and distributor.

I'd say the only way to know for sure if the superior power potential of a larger balancer offsets the quicker acceleration of a lightweight unit as installed in your car is by strip-testing them back to back. Assuming the lightweight unit in fact makes the car e.t. better, you still need to carefully inspect the bottom end when refreshing the engine for bearing cap walk, distortion, looseness, or fretting (material transfer). If the main-cap register's surface finish looks screwed up, you definitely have a problem. Cap fretting will cause uneven bearing loading, so a weird wear pattern on the bearings also is cause for concern.

If the engine is used in any long-duration closed-course racing, is installed in a boat, or is driven on the street, you should always use the biggest, stoutest balancer available. Assuming use of a factory balancer, the proper 8-inch unit designed for your performance engine offers the widest operating range and the best cancellation of unwanted resonant frequencies. As noted crank grinder Hank Bechtloff (aka Hank the Crank) puts it, "Even though it's more weight, the larger damper helps keep the frequencies that shake the crank apart from happening."

The preceding recommendations are for factory-style dampers. Most competition venues now require aftermarket SFI-certified dampers. The best professional-level, SFI-certified dampers-like ATI's Super Race dampers or TCI's Rattler series-come tuned for two different frequencies, as well as specific engine rpm operating ranges and strokes. They're high-buck, but the large-od versions are lighter than equivalent-size factory dampers, and the lighter and smaller-od versions are still suitable for nearly any application except the most extreme endurance usage.

Cryogenics, GM 6.5L Turbo Diesel Build
I am rebuilding a GM 6.5L turbodiesel with lowered compression, a larger turbo, and more fuel. How beneficial would it be to cryogenically treat the crankshaft? GM claims that the 6.5 nodular iron crank is as strong as a forged crank, but they have been known to break. I have heard and read that cryogenically treating engine components will increase strength 50 percent or more.

On a 2.4L twin-cam Olds engine, I noticed that a small hole was drilled in the crank end of the rod near the beam. I assume this is a piston squirter to help cool the piston crown. Would it be beneficial to do this to the 6.5 to keep piston temp down? GM installed piston squirters in the 6.5 in 1997 but had a lot of trouble with the blocks cracking. Assuming I can drill a squirter hole in the rods like the ones on the Olds, what would be the appropriate size hole?
Dave Rurey
Coeur d'Alene, ID

Back in the World War II days, scientists discovered that metals that had been frozen to ultralow temperatures had better wear characteristics and superior fatigue resistance. Over the ensuing decades, the science of cryogenic processing slowly evolved in fits and starts. Initially there were a lot of stability problems because the results were inconsistent and hard to control, with some parts even shattering outright. Years of refinement coupled with modern microprocessor-based industrial controls have finally overcome these stability problems.

4/9The 500hp race truck's twin turbos double the stock 15-psi boost pressure to 31 psi at max rpm. More streetable packages make from 320 to as high as 400 hp, up to double the typical stock motor's 190hp rating.

Today, nearly all rotating engine parts, engine blocks and heads, bearings, chassis parts, brakes, rearends, and even complete engine assemblies can be cryogenically treated. Joe Mondello now does cryogenic treating in-house. According to Mondello, the typical cryo process goes something like this: Before freezing the part, it's placed on a shaker table at ambient temperature to realign and normalize the part's molecular structure. The parts are then put in a cryo freezer where liquefied gases such as liquid nitrogen or liquid helium are used to cool down the part to (depending on the part) -250 to -320 degrees F for four to six hours. After the part comes out of the freezer, it is brought back to ambient temperature for one to two hours, then (if it previously hasn't been heat treated) baked at 275 to 300 degrees F for two to three hours.

"Shake and chill makes any part better," Mondello claims, confirming that "we have seen 50 percent strength increases, absolutely. There are 2,000hp big-block Chevys out there successfully running Chinese forged cranks that have been shaken and cryo'd."

5/9Heath prefers the small-bore 6.2L engines over the larger 6.5L mills because of their stouter pistons. Note the 6.2L piston's more robust wristpin bosses (right, compared with 6.5L piston, left). Pistons are available in 0.020-, 0.030-, 0.040-, and 0.060-inch oversizes for 6.2L and 6.5L engines.

A shaker table costs about $20,000 and the cryo freezer around $75,000, so in the past it cost an arm and a leg to get parts cryogenically treated. But Mondello says he's now able to shake and freeze a crank for around $275.

Looking at your 6.5L diesel engine's cast nodular iron crank specifically, cryogenics will make the crank much stronger, but-if you have a good casting-the stock part is plenty strong for your intended use; it has rolled fillets and is even nitrided from the factory. The problem is a small fraction of the cast cranks have random porosity. A porous crank can break without warning. Magnetic particle inspection will show surface flaws but cannot reliably detect voids or gas bubbles in the casting. The crank would need to be X-rayed (big bucks) to fully determine its structural integrity. And cryo won't fix a porous casting. The most cost-effective solution for getting a sufficiently strong, quality crank is Scat's Pro Stock 6.5L GMC diesel cast crank (Scat PN 965L3819); Summit sells it for around $420. If you're still worried, cryo the Scat unit.

6/9On the 6.2L piston (right) the ring groove is farther down from the piston deck, a superior location for a high-heat, high-boost application. By contrast, the 6.5L piston design (left) places the top ring closer to the piston deck to improve exhaust emissions-but at the expense of durability.

The reason a factory crank (assuming it passes X-ray) or the Scat crank is good enough for your use is that the GM '82 to '93 6.2L (379ci) and '92 to '00 6.5L (395ci) diesel engines' ultimate power potential is limited by the amount of fuel that can go through the production fuel pump (about 90mm³/injection stroke). The engine cannot make enough power to exceed the crank's strength limits. Unfortunately, no higher-capacity pump is available. In fact, due to the GM 6.2/6.5L diesel's peculiarities, your combination is going down the wrong path. To find the right path, I spoke to Bill Heath, the recognized expert on these engines who runs them in his land speed racing (LSR) truck.

First, Heath points out that in addition to porous factory cranks, other crank failures have occurred when the harmonic damper, damped serpentine beltdrive, and/or dual-mass flywheel have been used beyond a reasonable service life. If you plan to make serious power, they should be replaced with new units. Yet another common contributing factor to crank failure is a poorly machined main bearing main line. Heath says, "We believe that some blocks cure more completely after the machine work, resulting in a crooked main line and consequent crank failure."

7/9This is the business end of the 6.2/6.5's unique prechamber passage where it exits into the main combustion chamber. During the piston's compression stroke, nearly the entire cylinder's swept volume is forced at great velocity through this passageway into the prechamber where the fuel injector is located. This chamber design works best with a factory or higher compression ratio.

As to your hop-up plan, Heath stresses that the compression ratio should not be lowered. It's true that the typical turbocharged direct-injection diesel engine can successfully utilize lower compression to permit adding more fuel and boost without exceeding the particular engine design's dynamic compression-ratio and cylinder-pressure limitations. But the GM 6.2/6.5L is not a classic direction-injection diesel design. Instead, it utilizes a Ricardo-Comet swirl-type combustion chamber that relies heavily on close proximity of the piston to the head at TDC. The idea is to force most of the air volume in the main combustion chamber into the small, remote, prechamber prior to injection, relying on the resultant higher temperature and improved squish to yield more efficiency and power. Lowering compression on this type of chamber negatively affects the combustion process. On Heath's twin-turbo 500hp LSR truck, the engine's compression ratio is actually increased from the factory 21.3:1 to 22.5:1.

The 500hp, full-race, twin-turbo package is totally unstreetable because its bigger turbos completely ruin the bottom end. Nothing happens until 2,500 rpm. But if 320 hp is sufficient (as it is for the majority of Heath's customers who actually drive these engines on the street), you can retain the stock exhaust plumbing and single turbo configuration. A 320hp engine will peak around 3,700 rpm without chemical enhancements (no propane or nitrous), and the engine will still provide a long service life of towing and hauling. Heath offers its own high-output injectors, a custom engine controller and computer program, and a fully adjustable wastegate controller for use with the computer program.

Heath also offers marginally streetable packages up to 400 hp. That's more than double the typical stock 190hp rating. Note by 400 hp, you are starting to trade off long service life for hot rod performance, and at the 400hp level, custom stainless steel tube-type headers and larger turbos become mandatory.

8/96.2/6.5L engines use the same forged rods. The sturdy rods and heavy-wall pins are extremely robust. No trouble has been reported if the rods pass magnetic particle inspection, are properly rebuilt, and resized using new GM bolts. Pay particular attention to the wristpin bushings and clearance. Beam-polishing is unnecessary.

As with most engines, achieving higher-than-stock output in a package that lives requires rotating assembly enhancements. We've already covered the crankshaft. As for the pistons, Heath maintains, "On forged pistons the hard diesel rings will actually wear out the grooves. The stock pistons have a cast-in, wear-resistant, steel groove. They're a hypereutectic design made by Mahle. The 6.2L pistons are more robust than the 6.5L design, so we prefer to use the smaller-bore 6.2L engines to take advantage of the better pistons." The late 6.2L block, circa '91 to '93, casting No. 599, was also used to build the first 6.5L engines. Unlike earlier 6.2L blocks, No. 599 blocks have a one-piece rear main seal, making them compatible with the sturdier Scat crank. To differentiate which 599 block you have (6.2L or 6.5L), measure the bore size: 101 mm (3.976 inches) for a 6.2; 103 mm (4.055 inches) for a 6.5. You are also correct in avoiding the late blocks: The '97-and-later 6.5L blocks are prone to cylinder and main-web cracks.

The 6.2L's stock piston-to-deck clearance is on the big side, with the pistons 0.020 to 0.025 inch down in the whole at TDC. However, the Scat crank's stroke is 0.020 inch longer than the factory 6.2L crank's stroke. This will put the stock 6.2L piston about 0.010 higher at TDC, which is fine as it also gives a little more compression.

Heath does not use piston squirters. He just hasn't had any piston problems. "At our engine speeds, there's plenty of oil splashing around," he says. If you did want to drill holes in the rods to act as an oil squirter (about 0.040 inch is a good starting point), research would need to be conducted to make sure the holes are in the proper location to actually direct oil to the bottom of the piston for your engine design without weakening the rod. Such trial-and-error research is probably beyond the means of the average home builder. If piston life due to excessive heat becomes a problem, it may be easier to get the pistons high-tech coated. (Mondello gets $45/piston for ceramic top coatings, dry-lube skirt coatings, and oil-shedding bottom coatings.)

9/9Four-bolt main-bearing caps are stock on all five locations. To prevent cap walk or fretting, Heath uses its ARP fastener-based main stud kit on all its engines, preferring them to factory bolts or any girdle setup. Heath also uses ARP studs to retain the cylinder heads. Scat's cast crank is more durable than the stocker.

Also pay attention to the piston-to-cylinder wall clearance. You want the pistons tight enough so the cylinder walls can still pull heat out of the piston but not so tight the pistons scuff. Typical stock piston clearance specs are 0.0025 and 0.0047 inch for the 6.2 and 6.5, respectively. For the higher temperatures encountered in a hot rodded motor, Heath recommends increasing these values to 0.0032 and 0.0053.

One last durability issue is PMD (pump-mounted-driver) thermal stress-induced failures on '94 to '01 models. The PMD is screwed to the DS-4 injection pump, which is mounted in the intake valley, a high heat area. Heath Diesel offers a PMD isolator system that relocates the PMD behind the front bumper. It carries a seven-year warranty.